As one of techniques for implementing ring protection of an optical transmission system, BLSR (Bi-directional Line Switched Ring) has been put into practical use. Ina transmission system employing the BLSR, a plurality of nodes are connected by a pair of transmission lines in the form of a ring. The pair of transmission lines is configured to transmit a signal respectively in reverse directions. At normal operations, however, a data signal is transmitted via either of the pair of transmission lines. Namely, one of the pair of transmission lines is used as an active system, whereas the other is used as a standby system.
When a fault occurs in a transmission system employing the BLSR, node equipment autonomously switches a transmission route of a data signal so that a path for bypassing the fault is formed. As a result, the transmission system transmits the data signal by using not only a transmission line of an active system but that of a standby system.
In the BLSR, each of node equipments recognizes a network topology, which is connection information of the node equipments, in order to autonomously switch a transmission route when a fault occurs. The topology information that represents a network topology is generated, for example, by using node IDs uniquely assigned to node equipments. In the BLSR, a function of detecting a fault and automatically recovering a path is sometimes referred to as APS (Automatic Protection Switching).
As a related technique, a ring network setup method for automatically setting up a topology in a ring network where a plurality of transmission equipments are connected by transmission lines in the form of a ring is proposed. This method includes a procedure of setting, in all transmission equipments, one or a plurality of bytes for transferring topology information within an overhead of a transmission frame, a direction where the topology information is transferred by the bytes, and a topology setup completion decision condition, and a procedure of sequentially transmitting the topology information by the set bytes in the set transfer direction from designated transmission equipment, and of determining whether or not a topology setup has been complete according to the topology setup completion decision condition (for example, Japanese Laid-open Patent Publication No. 2004-214826).
As another related technique, optical transmission equipment used in a ring transmission system where a time-division multiplexed optical signal can be bidirectionally transmitted is proposed. This optical transmission equipment includes connection form recognition means, connected to a bidirectional ring transmission line, for recognizing a connection form between a transmission ring and another transmission ring connected to the transmission ring, fault section detection means, connected to the connection form recognition means, for detecting a section where a fault has occurred, and loop-back switching control means, connected to the connection form recognition means and the fault section detection means, for switching a transmission route based on the connection form and the section (for example, Japanese Laid-open Patent Publication No. 2006-129530).
In BLSR used in SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy), a maximum of 16 nodes can be arranged in one ring network. In recent years, however, the demand for arranging a lot more nodes in one ring network has been increasing. Accordingly, for example, recent OTN (Optical Transport Network) architectures recommend that a maximum of 128 or 256 nodes can be arranged in one ring network.
Incidentally, different node IDs are assigned to nodes in order to implement BLSR. However, when the number of nodes arranged in one ring network increases, it becomes difficult to assign different node IDs to the nodes, and to manage the node IDs.
For example, when node equipment is newly added to a ring network, an operator who sets up or manages the network initially verifies node IDs of node equipments on the ring network. The operator searches for and obtains an unused node ID. Then, the operator assigns the obtained node ID to the new node equipment. Accordingly, if the number of nodes arranged in one ring network grows, the length of time needed to assign and manage node IDs increases. Additionally, setting mistakes such as a mistake of redundantly assigning the same node ID to a plurality of node equipments are more prone to occur.
This problem occurs not only in a transmission system employing BLSR. Namely, the above described problem may occur in a ring network where a plurality of transmission equipments are connected by transmission lines in the form of a ring.